The biological behavior of naturally occurring tumors in cats and dogs is still unknown. One of the objectives of this work is to determine the biological behavior of a variety of tumor types and to study the effectiveness of various therapy regimes, especially those employing immunotherapy on these tumors. The effectiveness of surgery alone and combined with levamisole immunotherapy for feline and canine mammary tumors is being compared. Canine oral melanomas are being treated with cryosurgery, alone or combined with C. parvum immunotherapy, and dogs with lymphosarcoma are beng treated with chemotherapy either alone or together with levamisole immunotherapy. Healthy feline leukemia virus exposed or infected cats are being treated with levamisole in order to determine if this form of immunotherapy is able to augment the cat's immune system resultng in rejection of the virus or prevention of disease. In each of these research programs it is still too early to reach any conclusions about the relative effectiveness of the therapy regimes that are being compared. In another program we have discovered that blood constituents (whole blood, serum, or plasma) from healthy cats and dogs have an anti-leukemic activity. We are now attempting to elucidate the mechanisms responsible for this activity and have succeeded in partially purifying the anti-leukemic factor using a Ca2PO4 gel.